Fix marshalling causing crashes with using the -fork flag.

[WINSDK]

When testing on MacOS I was running my benchmarks with the -fork flag.
This causes every benchmark to throw an exception with:

Uncaught exception:
  
  (Invalid_argument "output_value: functional value")

Raised by primitive operation at Stdlib__Marshal.(partial) in file "marshal.ml" (inlined), lines 28-29, characters 0-93
Called from Core_bench__Benchmark.measure_all.(fun) in file "src/benchmark.ml", line 232, characters 12-38
Called from Stdlib__List.iter2 in file "list.ml" (inlined), line 160, characters 24-31``

It appears that when using -fork, each measurement is marshalled over a pipe.
However measurements have samples and their samples include an extra field which can't be marshalled.

match Caml_unix.fork () with
| 0 ->
  let x = measure run_config test in
  let open Stdlib in
  let oc = Caml_unix.out_channel_of_descr fdw in
  Marshal.to_channel oc x []; (* This fails to marshall. *)
  exit 0
| pid -> ignore (Caml_unix.waitpid [] pid : int * Caml_unix.process_status))

I've changed the samples using a Map to just using an association list which fixes the issue.